Inkjet is a non-impact dot-matrix printing technology in which droplets of ink are jetted from a small nozzles directly to a specified position on a media to create images or characters. Inks are generally classified into two kinds: inks for coating layer (such as: white ink, transparent ink or other bottom color inks) and colored inks (using for printing images or characters); white ink is generally employed to print background color on non-white printing medium to improve vividness of the printed images, or to print foreground color on transparent printing medium to form reflected images. When printing background images, firstly ejecting white ink on non-white printing medium to form white background, colored images are then printed on the white background, and final images are viewed from the printing side; when printing foreground images, colored images are firstly printed on the transparent printing medium, white ink is then printed on the colored images, and final images are viewed by transmitting the transparent printing medium. Because, colored inks are usually, to some extent, transparent, printing on colored printing medium usually makes the images look like somewhat faded, consequently, a layer of completely opaque white ink is usually printed on the whole printing area of the non-white printing medium, afterwards, printing images on the printing medium already printed by white ink to guarantee brilliance of the images. When printing white ink, in order that white ink evenly covers the whole printing area, demands on print resolution of white ink is high, and its print resolution is usually higher than that of the printed images, such as 1440*1440 dpi. In order to prevent the printed colored images from shedding in prior art or increase the brightness of the printed colored images, a transparent protective layer usually covers the colored images.
In prior art, two sets of UV solidification devices are respectively installed in each side of the carriage; in the course of printing, due to the overlying of bottom color and printed images, when printing images, solidification of ink droplets of the bottom color is necessary, otherwise, ink droplets of different colors would be mixed together, and printing quality be affected. Therefore, width of UV solidification devices must illuminate the whole printing breadth, i.e., width of UV solidification device should be equal to or greater than the total width of the print heads positioned on the carriage's baseplate; while, the wider the area illuminated by the UV solidification devices, the more expensive the UV solidification devices are. As regards to scanning inkjet printing apparatus, in prior art, there are mainly two ways to print white backdrop: in one, white ink is printed firstly, after printing of white ink, printing medium is reversed back, then colored images are printed on the already finished white ink backdrop, arrangement of print heads in this method is shown in FIG. 2, all the print heads are arranged side by side along one axis, in this method, precision of paper feeding direction is not that accurate, printing accuracy of paper feeding direction is hard to be guaranteed, and printing time is accordingly prolonged; in the other method, print heads ejecting white ink and those ejecting colored inks are arranged vertically, print heads ejecting white ink are located at the paper inlet end of the printing medium, printing medium is firstly printed by white ink, then colored images are printed on printing medium already printed by white ink, this method is suitable for printer with relatively few rows of print heads, as shown in FIG. 3, there are two rows of print heads, print head ejecting ink for coating layer in one row, and those ejecting colored inks in the other row, and each print head is specifically for each ink color; in industrial printing, printing output is usually improved by connecting several print heads in series, as in FIG. 4 (where a specific ink is ejected by two print heads), the two print heads ejecting ink for coating layer are connected in series in a row, and print heads ejecting colored inks are connected in series in the other row, relatively large number of print heads leads to the baseplate where print heads are located widened, correspondingly, demands on width of ink solidification devices are also raised, when there are three rows of pint heads in the vertical direction, a UV lamp can illuminate the whole printing area, if there are four rows of print heads in the vertical direction as in FIG. 4, two UV solidification devices or a much wider UV solidification device are (is) needed to illuminate the whole printing area, price of UV solidification device is very expensive and, this makes printing cost greatly increased, meanwhile, motion stability of the carriage during the printing process is also reduced due to widened print head baseplate on which located the print heads.